Denudation outpaced by crustal thickening in the eastern Tianshan

The modern high topography of the Tianshan resulted from the reactivation of a Paleozoic orogenic belt by the India/Asia collision. Today, the range exhibits tectonically active forelands and intermontane basins. Based on quantitative morphotectonic observations and age constraints derived from cosmogenic 10Be dating, single-grain post-infrared infrared stimulated luminescence (p-IR IRSL) dating and modeling of fault scarp degradation, we quantify the deformation in the Nalati and Bayanbulak intermontane basins in the central Eastern Tianshan. Our results indicate that at least 1.4 mm/yr of horizontal crustal shortening is accommodated within these two basins. This shortening represents over 15% of the 8.5 ± 0.5 mm/yr total shortening rate across the entire range at this longitude. This shortening rate implies that the Eastern Central Tianshan is thickening at a mean rate of ∼1.4 mm/yr, a rate that is significantly higher than the average denudation rate of 0.14 mm/yr derived from our cosmogenic analysis. This discrepancy suggests that the Tianshan range has not yet reached a steady-state topography and remains in a transient state of topographic growth, most likely due to limited denudation rates driven by the arid climate of Central Asia

Les déséquilibres radioactifs 238U-230Th-226Ra : discussions sur les sources et processus responsables du volcanisme de la Cordillère des Andes et sur la déglaciation en Islande

Thanks to precise analyses of uranium series radioactive disequilibria, permitted by improvements of mass spectrometry data acquisition and optimization of chemistry, two questions were tackled and answers about the periods of glaciation in Iceland and the petrogenesis of andean lavas were proposed.Two methods were used to date lava flows from the Reykjanes peninsula and the island of Heimaey, in Iceland, directly linked to the periods of deglaciation or reheating around the last ice age. An original method based on the use of segregation veins present in the lava flows has been developed. Thanks to the formation of this particular geological object this methods permits to be freed from xenocrysts when using the internal isochron method in the 238U-230Th system. Using this method some constraints are brought both on the quaternary geology of the area of Reykjavik and on the age of the end of the Würm glaciation on Heimaey. In parallel, due to the homogeneity of the historical and Holocene lavas of the Reykjanes peninsula, regarding the (230Th/232Th) and Th/U ratios, it is possible to derive the (230Th/232Th) ratio in time to determine the age of lava flows installed during the ice age. Thus we confirmed that the magnetic excursion recorded in the Skálamælifell lava flow is most probably the same one as that of Laschamp-Olby and that at this time (approximately 48.000 years) the Reykjanes peninsula was entirely covered with a glacier. Thanks to this method the extent of the glacier was also constrained by dating of a picritic shield around 22.000 years, thus the peninsula has to be free of any ice cap despite the fact that glaciation is the most intense around this time.Then andean arc magmas petrogenesis was constrained in time and space. Almost systematic excesses of 226Ra compared to 230Th in lavas of austral, southern and northern volcanic zones of the Andes (AVZ, SVZ and NVZ) constrain the ascending time from the source to the surface after their formation which is necessarily fast (less than 8.000 years to preserve these disequilibria). We show that eruptive activity and excesses of 238U (and to a lesser extent 226Ra) compared to 230Th are linked. This observation suggests that the most active volcanoes (Villarrica and Llaima for the SVZ) are those whose source is enriched in fluids, coming from the subducted plate and sediments which cover it, during partial melting of the mantle wedge and that radioactive disequilibria tell us about the geodynamic context on a regional scale. Finally a more precise study of particular volcanoes (Nevado de Longavi (SVZ) and Guagua Pichincha (NVZ)), suggests that uranium series disequilibria record the evolution of a volcano in time. Thus the complex history of Nevado de Longavi and the particular signals of this volcano can be translated in term of variation of partial melting degree of the source, fractional crystallization or crustal contamination. In the case of Guagua Pichincha volcano, the adakitic source of the volcanic activity could be discussed thanks to the study of 238U and 226Ra excesses compared to 230Th and a model of adakites formation by partial melting of an hydrated mantle wedge followed by fractional crystallization of garnet at high pressure will be proposed.Par le biais d'analyses précises des déséquilibres radioactifs des séries de l'uranium, permises par des améliorations effectuées aussi bien au niveau de l'acquisition des données par spectrométrie de masse qu'au niveau de la qualité de la chimie, deux grandes questions ont été abordées et des réponses au sujet des périodes de glaciation en Islande et la pétrogenèse des laves Andines ont été proposées. Deux méthodes ont été utilisées pour permettre la datation de coulées de laves de la péninsule de Reykjanes et de l'île d'Heimaey, en Islande, directement en rapport avec les périodes de déglaciation ou de réchauffement autour de la dernière période glaciaire. Une méthode originale basée sur l'utilisation des veines de ségrégation présentes dans les coulées de laves a été mise au point. Cet objet géologique particulier permet de s'affranchir des xénocristaux lors de l'application de la méthode de l'isochrone interne dans le système 238U-230Th, du fait même de son mode de formation. Grâce à cette méthode et à son utilisation sur une coulée de lave de la région de Reykjanes et une coulée au nord de l'île d'Heimaey des contraintes sont apportées à la fois sur la géologie quaternaire de la région de Reykjavik et sur l'âge de la fin de la glaciation de Würm sur l'île d'Heimaey. En parallèle, l'homogénéité des laves historiques et holocènes de la péninsule de Reykjanes, en ce qui concerne les rapports (230Th/232Th) et Th/U, a permis de dériver dans le temps le rapport (230Th/232Th) pour déterminer l'âge de coulées datant de la période glaciaire. Ainsi il a été confirmé que l'excursion magnétique enregistrée dans la coulée de Skálamælifell est très probablement la même que celle de Laschamp-Olby et qu'à cette époque (environ 48.000 ans) le glacier, recouvrait la péninsule de Reykjanes. Par cette même méthode l'étendue du glacier a également été contrainte par la datation d'un bouclier de picrite aux environs de 22.000 ans, âge, pourtant en pleine glaciation, où la péninsule devait être libre de toute chape de glace.Dans un deuxième temps, la pétrogenèse des laves de l'arc Andin a été contrainte dans le temps et l'espace. Des excès quasiment systématiques de 226Ra par rapport à 230Th dans les laves des zones volcaniques australes, sud et nord des Andes (AVZ, SVZ et NVZ) nous renseignent sur le temps de remontée des magmas après leur formation qui est nécessairement rapide (moins de 8.000 ans pour conserver ces déséquilibres). Un lien a également pu être fait entre l'activité éruptive et les excès de 238U (et dans une moindre mesure 226Ra) par rapport à 230Th. Cette observation suggère que les volcans les plus actifs (Villarrica et Llaima pour la SVZ) sont ceux dont la source est la plus enrichie en fluides, provenant de la plaque subduite et des sédiments qui la couvrent, lors de la fusion partielle du coin de manteau et que les déséquilibres nous renseignent sur le contexte géodynamique à l'échelle régionale. Enfin l'étude plus fine de volcans particuliers (Nevado de Longavi (SVZ) et Guagua Pichincha (NVZ)), suggère que les déséquilibres radioactifs des séries de l'uranium enregistrent l'évolution d'un volcan. Ainsi l'histoire complexe du Nevado de Longavi peut être appréhendée par cette approche et les signaux particuliers de ce volcan peuvent être traduits en terme de variation de fusion partielle de la source, cristallisation fractionnée ou contamination crustale. Dans le cas du Guagua Pichincha c'est la source du volcanisme adakitique qui pourra être discutée grâce à l'étude des excès de 238U et 226Ra par rapport à 230Th et un modèle de formation d'adakites par fusion partielle du coin de manteau hydraté puis cristallisation de grenat à haute pression sera proposé

A NEW COMPILATION OFELEMENT CONCENTRATIONS INTHE NATURAL RIVER WATERSTANDARD SLRS-6 (NRC-CNRC)

International audienceIn Geosciences, there are only few natural river watercertified reference materials to ensure the quality of theroutine consentration's determination. One of thoseCRM is provided by the NRC-CNRC and called SLRS.Actually, the SLRS-6 has been released. However,there is only 19 elements having their concentrationcertified. Those elements are used to validate themeaasurements. Therefore, when a larger investigationis done, which is the case for the laboratories implied inthe CNRS "Isotrace" consortium, it is relativelylimitant.As done for the previous version of the SLRS (4-5) ourconsortium, working on major and trace elements innatural water, studied in details the CRM SLRS-6.They validated the measurement on the 19 certifiedelements and went further by determining other noncertifiedelements based on regular analysis of themusing ICP-MS and ICP-OES.We now offer a compilation of values for about 20trace elements, including Rare Earth Elements, whichare not certified by the producer as well as for silicon.Values are determined using robust mean and theirassociated expanded uncertainties. The Sr isotopic ratiois also proposed.Thi

Extreme silicon isotope fractionation due to Si organic complexation: Implications for silica biomineralization

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The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

International audienceThis study provides experimental evidence of the resetting of the magnesium (Mg) isotope signatures of hydromagnesite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over month-long timescales. All experiments were performed in batch reactors in aqueous sodium carbonate buffer solutions having a pH from 7.8 to 9.2. The fluid phase in all experiments attained bulk chemical equilibrium within analytical uncertainty with hydromagnesite within several days, but the experiments were allowed to continue for up to 575 days. During congruent hydromagnesite dissolution, the fluid first became enriched in isotopically light Mg compared to the dissolving hydromagnesite, but this Mg isotope composition became heavier after the fluid attained chemical equilibrium with the mineral. The δ26Mg composition of the fluid was up to ∼0.35‰ heavier than the initial dissolving hydromagnesite at the end of the dissolution experiments. Hydromagnesite precipitation was provoked during one experiment by increasing the reaction temperature from 4 to 50 °C. The δ26Mg composition of the fluid increased as hydromagnesite precipitated and continued to increase after the fluid attained bulk equilibrium with this phase. These observations are consistent with the hypothesis that mineral-fluid equilibrium is dynamic (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium). Moreover the results presented in this study confirm (1) that the transfer of material from the solid to the fluid phase may not be conservative during stoichiometric dissolution, and (2) that the isotopic compositions of carbonate minerals can evolve even when the mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests that the preservation of isotopic signatures of carbonate minerals in the geological record may require a combination of the isolation of fluid-mineral system from external chemical input and/or the existence of a yet to be defined dissolution/precipitation inhibition mechanism

Weak impact of landscape parameters and rock lithology on Mg isotope composition of the Yenisey River and its tributaries

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Performances of 800 nm femtosecond laser ablation on natural and synthetic quartz

International audienceA commercial femtosecond laser system operating at its fundamental wavelength (l ¼ 800 nm, near Infra-Red) was used to ablate both synthetic and natural quartz on polished and unpolished surfaces. Ablation rates and maximum depths were determined using two distinct optical setups: a 25 mm focal length Cassegrain reflecting objective, and a 50 mm focal length convergent coated lens. All samples were ablated with the same laser beam at E 0 ¼ 1 mJ, t ¼ 60 fs, f ¼ 5 Hz and N ¼ 10-8000 shots. The depth of ablation craters obtained with the lens shows a linear increase with shot number N up to N ¼ 2000 shots. Then the depth increases much less with N and reaches a plateau above N ¼ 3000 shots. Maximum depth was close to 1300 mm for N ¼ 3000 shots. Using the reflecting objective, ablation rate starts from 0.42 mm/shot and decreases rapidly to 0.02 mm/shot at a maximum depth of 350 mm for N ¼ 1500 shots. Ablation thresholds (F th) were calculated for 1 and 10 consecutive shots with energy increasing from E 0 ¼ 0.1-2 mJ/ pulse. Threshold values varies from F th ¼0.1 J.cm À2 (unpolished, 10 shots) to F th ¼ 2.9 J.cm À2 (polished, single shot). The energy penetration of IR-femtosecond laser pulses in quartz has been calculated at l ¼ 271 nm. The low absorption of IR wavelengths in quartz affects the ablation efficiency in the first shots. The associated non-linear effects are visible on a crater FIB foil observed with TEM as progressive high-pressure photomechanical damage developing under the ablation pit. The present study emphasizes the potential of IR-femtosecond laser for ablation of highly transparent material, and provides reliable data for LA-ICP-MS applications in earth sciences

In situ characterization of infra red femtosecond laser ablation in geological samples. Part B: the laser induced particles

International audienceThe analytical study of infra red femtosecond laser induced particles has been performed using Transmission Electron Microscopy (TEM) and Low Pressure Impaction (ELPI). Various natural and synthetic matrices have been tested: monazite (phosphate), zircon (silicate), NIST610 (glass), spinel (oxide), quartz (silicate structure), silicon (semiconductor) and Nordic gold (metallic alloy). Three types of particles are systematically observed: very rare large round spherical particles (d p z 1 mm) whose composition is close to the initial sample, spherical particles of smaller size (d p # 250 nm) and agglomerates of d p z 10 nm particles. Chemical compositions of the latter two are complementary with respect to the ablated sample. Isolated occurrence of hydrodynamic sputtering may explain the creation of rare large droplets. Other particles are probably generated from the irradiated matter in the supercritical state during the cooling process and plasma expansion. A recent model provides a strong basis to describe vapour to particle conversion and further condensation/coalescence processes for simple systems (single component). Additional assumptions must be included to apply the model to our observations of complex multi-elemental systems. A qualitative interpretation may be proposed on the basis of fractionated condensation/coalescence and further agglomeration of particles, depending on plasma pressure and the ablated elements properties (mainly density and melting period) as well as the thermal evolution of the plume. This interpretation is discussed and validated for each sample type. Previous results concerning ablation mechanisms using the same system are included in our model. The generation of particles from a vapour phase confirms that vaporization is the main ablation mechanism in the femtosecond regime. Moreover, the possible presence of molecular sized clusters in the initial plasma, which can accelerate the nucleation process, strongly suggests that fragmentation is the secondary ablation mechanism. Finally, the present study is an experimental validation for recent femtosecond laser ablation simulation, and it brings new insights for interpreting particles generation processes for complex systems. Correlations between laser ablation ICP-MS measurements must now be made with the present results

In situ characterization of infrared femtosecond laser ablation in geological samples. Part A: the laser induced damage

International audienceInfrared femtosecond laser induced damage has been studied in order to determine, with analytical protocols, the processes involved in laser ablation in this regime. Transmission Electron Microscopy (TEM) coupled with Focused Ion Beam (FIB) milled cross-sections of natural ablated monazite were used. Craters were formed using N = 1 and 3 shots, E0 = 0.1 and 0.8 mJ per pulse and τ = 60 fs. Observations revealed that laser settings induce little changes in the nature and size of damaged structures. The crater bottom forms a ∼0.5 μm layer composed of melted and recrystallized monazite grains, and spherical ∼10 nm voids. The underlying sample shows lattice distortions, progressively attenuated with depth, typical of mechanical shocks (thermoelastic relaxation and plasma recoil pressure). No chemical difference appears between these two domains, excluding preferential vaporization and thus laser induced chemical fractionation. Correlations with existing molecular dynamics (MD) simulations indicate that the deep distorted lattice probably undergoes spallation whereas the upper layer rather goes through homogeneous nucleation. Nevertheless, these processes are not pushed forward enough to induce matter removal in the present conditions. In consequence, photomechanical fragmentation and vaporization, requiring higher energy density states, would rather be the main ablation mechanisms. This hypothesis was supported by an additional study focused on the laser produced aerosols. Further links to LA-ICP-MS measurements can then be developed